On the Origin of the Electric Organ in Malapterurus Electricus

Abstract

The present study was made on two small specimens of Malapterurus electricus, of standard lengths 11.4 and 12.7 mm. As is well known, the postembryonic growth of electric organs in Malapterurus and other electric fishes takes place by an enlargement of the electric units and not by an increase in the number of the electric plates. In the present material, however, there is a multiplication of electric tissue elements in the rostral portion of the electric organ. The structure of this multiplication zone is described. In the anterior region the connective tissue membranes which surround the two halves of the electric organ form structures similar to tendons which are fixed to the ventral surface of the shoulder girdle on each side of the median line. A small deficiency on each side in the muscular wall in the same region was observed in adult specimens by Maurer. This deficiency is more evident in the young specimens studied in the present paper and it is covered from the outside by the multiplication zone of the electric organ. In this place the electric nerve joins the electric organ. The giant electric cell, the surface of which is penetrated by intracellular capillaries, is situated in the anterior part of the spinal cord and its axon emerges with the third ventral spinal root. In the multiplication zone the connective tissue membranes are completely independent of the dermal connective tissue and the space between the electric organ and the skin is of a conventional subdermal type. These circumstances strongly indicate that the electric organ is of myoblastic origin in Malapterurus as in all other electric fishes known in this respect. Nothing has been observed which supports the idea of an adenoid origin of the electric organ in Malapterurus.